Rigid polyurethane foams are used in numerous industries. They are produced by reacting an appropriate polyisocyanate and an isocyanate-reactive compound, usually a polyol, in the presence of a blowing agent. One use of such foams is as a thermal insulation medium in the construction of refrigerated storage devices. The thermal insulating properties of closed-cell rigid foams are dependent upon a number of factors, including the average cell size and the thermal conductivity of the contents of the cells. Chlorofluorocarbons (CFC's) were typically used as blowing agents to produce these foams because of their exceptionally low vapor thermal conductivity. However, CFC's are now known to contribute to the depletion of ozone in the stratosphere and, as a result, mandates have been issued which prohibit their use.
Initially, the most promising alternatives to CFC's appeared to be hydrogen-containing chlorofluorocarbons (HCFC's). While HCFC's, such as HCFC 141b, have been used as alternatives to CFC's, they have also been found to have some ozone-depletion potential. There is, therefore, a desire to find substitutes for HCFC's as well as CFC's.
Alternative blowing agents, such as hydrofluorocarbons (HFC's), also replaced CFC's. The compounds are, however, greenhouse gases. Hydrocarbons, such as pentane isomers, have also been used but these are flammable and have lower energy efficiency. Halogenated hydroolefinic compounds, such as hydrofluoropropenes and/or hydrochlorofluoropropenes, are possible candidates as replacements for HFCs, since their chemical instability in the lower atmosphere provides for a low global warming potential and zero or near zero ozone depletion properties.
A drawback to some of these materials as blowing agents in the production of satisfactory isocyanate-based foams, however, is shelf-life issues. Blowing agents usually are combined with polyols and other components, such as surfactant(s) and catalyst(s) to form a so-called “B-side” pre-mix that may be stored for up to several months prior to being combined with an “A-side” isocyanate component to form the foam. A drawback of systems that use certain hydrohaloolefins is the shelf-life of the B-side composition. If the B-side composition is aged prior to combining with the polyisocyanate, the foam can be of lower quality and may even collapse during the formation of foam. The poor foam structure is thought to be attributable to the reaction of certain catalysts with these hydrohaloolefins which results in the partial decomposition of the blowing agent and, as a result, undesirable modification of silicone surfactants, resulting in poor foam structure and quality.
Foam-forming compositions used in the production of discontinuous panel insulation, such as insulation for refrigerated trailer side-walls and roofs, must exhibit a stringent combination of properties. For example, in addition to good thermal insulation properties, they must exhibit a target gel time conducive to the manufacturing equipment and process that is used, and they must exhibit a long shelf life, which means that this gel time cannot change by a large amount after storage of the foam-forming composition components for a long period of time (several months or more), even when water is used as a blowing agent component in an amount of 2% by weight or more, based on the total weight of the isocyanate-reactive component. They also must exhibit good dimensional stability (low foam shrinkage) even when the foams have a relatively low core foam density of 1.4 to 1.7 lb/ft3 (22.4 to 27.2 kg/m3). In addition, it is important that the foam adheres well to facer substrates typically used in refrigerated trailer side-wall and roof applications. A composition that can fulfill most, if not all, of these requirements, while utilizing a halogenated hydroolefin blowing agent would, therefore, before highly desirable.